1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, a backlight unit using light emitting diodes and a liquid crystal display device using the same.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, various types of flat panel displays are being researched and developed as a substitute for CRTs such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs). Of these flat panel displays, LCD devices have many advantages, such as high resolution, light weight, thin profile, compact size, and low voltage power supply requirements.
In general, an LCD device includes two substrates that are spaced apart and face each other with a liquid crystal material interposed between the two substrates. The two substrates include electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal material. Alignment of the liquid crystal molecules in the liquid crystal material changes in accordance with the intensity of the induced electric field and the direction of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field.
The LCD devices need a backlight unit to supply light. A light source may include a CCFL (cold cathode fluorescent lamp), an EEFL (external electrode fluorescent lamp) and an LED (light emitting diode). Among theses light sources, the LED has many advantages, such as low power consumption, high reliability and compact size.
FIG. 1 is a cross-sectional view illustrating an LCD device according to the related art. Referring to FIG. 1, the LCD device according to the related art includes a liquid crystal panel 10, a backlight unit 20, a main supporter 30, a bottom case 50 and a top case 40. The liquid crystal panel 10 includes first and second substrates 12 and 14 and a liquid crystal layer between the first and second substrates 12 and 14. The backlight unit 20 is below the liquid crystal panel 10. The backlight unit 20 includes a reflector 25 on the bottom case 50, a light guide plate 23 on the reflector 25, an LED assembly 29 along a side of the light guide plate 23, and a plurality of optical sheets 21.
The LED assembly 29 includes a PCB (printed circuit board) 29b and a plurality of white LEDs 29a installed on the PCB. White light emitted from the white LEDs enters the light guide plate 23 at an edge and is refracted in the light guide plate 23 toward the liquid crystal panel 10. The refracted light along with light reflected by the reflector 25 pass through the plurality of optical sheets 21 to produce a high quality plane of light that is supplied to the liquid crystal panel 10.
The liquid crystal panel 10 and the backlight unit 20 are accommodated in the main supporter 30 having a frame shape, and the main supporter 30 is coupled with the top cases 40 to cover peripheral portions of the liquid crystal panel 10, and the bottom case 50 covers the bottom of the backlight unit 20. These components are assembled into the LCD device. First and second polarizing plates 19a and 19b are attached on outer surfaces of the first and second substrates 12 and 14, respectively.
FIG. 2 is the CIE color coordinates illustrating a color distribution of an example of a white LED. In FIG. 2, the CIE color coordinates are referred to as 1931 CIE color coordinates.
The white LED includes a blue LED chip and a yellow phosphor covering the blue LED chip. The yellow phosphor is made of a YAG (Yttrium Aluminum Garnet) group and a TAG (Terbium Aluminum Garnet) group, or a silicate group. Accordingly, the white LED emits white light by mixing a blue light emitted from the blue LED chip and a yellow light emitted from the yellow phosphor. Generally, it is desired that the LEDs are manufactured to have the same color property that is appropriate for the light source of the backlight unit. However, the color properties of the white LEDs, in reality vary due to a difference in the dominant wavelength of the blue LED chip and differences in the content of the yellow phosphor among the white LEDs manufactured. Accordingly, referring to FIG. 2, even though the white LEDs emit white light, the white light emitted from the white LEDs vary in color but have a distribution to some extent in the color space.
The white region shown in FIG. 2 is divided into a plurality of regions. These regions are referred to as bins, and the process of dividing the white LEDs is referred to as a binning. An LCD device manufactured by a manufacturer needs to produce a target white light that is associated with a specific bin, for example, a target bin TB that has an appropriate color property for its own LCD device among the plurality of bins. Accordingly, the LEDs belonging to other bins are not used for the LCD and wasted. Even though the LCD manufacturer uses different bins within a tolerance for error for LCD its devices, white is different between LCD devices due to using the different bins. Accordingly, because only a small number of the white LEDs manufactured is utilized for the LCD device, production efficiency of the white LED is reduced and production cost thus increases. Therefore, production cost of the backlight unit and LCD device increases.